For increasing fuel efficiency typically in automobiles and transports (transport equipment), weight reduction of automobiles and transports are demanded. Typically, it is effective for weight reduction to employ high-strength steel sheets so as to allow parts constituting the automobiles and transports to have smaller thicknesses. In addition, automobiles particularly require collision safety, and structural parts such as pillars, and reinforcing parts such as bumpers and impact beams should therefore have further higher strengths. However, steel sheets, if having a higher strength, have poor ductility (hereinafter also referred to as “elongation capacity” or “elongation”) and thereby have inferior workability. Such high-strength steel sheets should have both a high strength and good workability (good balance between tensile strength (TS) and elongation (EL)).
As a technique for obtaining a high-strength steel sheet having both a high strength and good workability, for example, U.S. Patent Application Publication No. 2008/0178972 proposes a high-strength steel sheet which has a structure including martensite and retained austenite as second phases being dispersed in specific proportions in ferrite matrix and which excels in elongation and stretch flangeability.
U.S. Patent Application Publication No. 2009/0053096 proposes a high-strength cold-rolled steel sheet which has controlled contents of silica (Si) and manganese (Mn), has a structure including tempered martensite and ferrite as principal components and further including retained austenite, and excels in coating adhesion and elongation.
Japanese Unexamined Patent Application Publication (JP-A) No. 2010-196115 proposes a high-strength cold-rolled steel sheet which has a structure including ferrite, tempered martensite, martensite, and retained austenite and excels in workability and impact resistance.
Japanese Unexamined Patent Application Publication (JP-A) No. 2010-90475 proposes a high-strength steel sheet which has a structure including bainitic ferrite, martensite, and retained austenite, excels in elongation and stretch flangeability, and has a tensile strength of 980 MPa or more.
Recent steel sheets typically for automobiles particularly require improvements not only in the proposed properties such as strength and workability but also in safety in assumed use environments. For example, the steel sheets are demanded to have also satisfactory resistance to cold brittleness, on the assumption of body collision under low-temperature conditions during wintertime. However, the customary steel sheets, which are intended to improve strength and workability, fail to ensure sufficient resistance to cold brittleness, because they tend to have inferior resistance to cold brittleness when having higher strengths. Thus, further improvements have been demanded.